Many electronic and input devices include a touch-sensitive surface for receiving user inputs. Devices such as smart telephones, tablet computing devices, laptop computers, track pads, wearable communication and health devices, navigation devices, and kiosks can include a touch-sensitive surface. In some cases, the touch sensitive surface is integrated with a display to form a touch-screen or touch-sensitive display.
The touch-sensitive surface may detect and relay the location of one or more user touches, which may be interpreted by the electronic device as a command or a gesture. In one example, the touch input may be used to interact with a graphical user interface presented on the display of the device. In another example, the touch input may be relayed to an application program operating on a computer system to effect changes to the application program.
Touch-sensitive surfaces, however, are limited to providing only the location of one or more touch events. Moreover, touch, like many present inputs for computing devices, is binary. The touch is either present or it is not. Binary inputs are inherently limited insofar as they can only occupy two states (present or absent, on or off, and so on). In many examples, it may be advantageous to also detect and measure the force of a touch that is applied to a surface. In addition, when force is measured across a continuum of values, it can function as a non-binary input.
Various sensing technologies can be used to detect force, such as capacitive, resistive, ultrasonic, and magnetic technologies. With some resistive technologies, force is determined by detecting changes in resistance between multiple strain sensing elements. In some devices, it is desirable to have the resistances of the strain sensing elements match one another. But differences in the resistances can be caused by variations in fabrication processes, such as variations in alloy ratios, annealing, the deposition temperature, oxygen incorporation, and/or sputtered thickness. In some circumstances, the resistance variations are random. Some of the resistance variations occur over the entire force sensor, while other resistance variations may be more localized. In some situations, mismatches between the resistances can contribute to common mode offset.